It is well known in the art that anodized or photoanodized-created porous semiconductors exhibit unique optical properties which can not be matched by their bulk semiconductor counterparts. For example, porous silicon (Si) exhibits high efficiency luminescence above the 1.1 eV band-gap of bulk silicon, which enables optical devices to be fabricated from porous Si. In another example, porous alpha-silicon carbide (6H--SiC) has superior optical and unique electronic properties due to its geometry and resulting band structure, including visible transparency, intense blue photoluminescence and electroluminescence as compared with bulk SiC, which is inferior by way of the indirect nature of bulk SiC's band-gap that limits the efficiency of bulk SiC-based optoelectronic devices. In the case of SiC, by electrochemically fabricating a microcrystalline porous structure with pore spacings of "quantum" dimensions (less than 10 nm) which provides a large internal surface area using either dark-current or light-assisted electrochemical means as disclosed in U.S. Pat. No. 5,376,241 entitled FABRICATING POROUS SILICON CARBIDE and U.S. Pat. No. 5,454,915 entitled METHOD OF FABRICATING POROUS SILICON CARBIDE (SIC), both of which were issued to Joseph S. Shor and Anthony D. Kurtz on Oct. 3, 1995 and assigned to Kulite Semiconductor Products, Inc. the assignee herein, it is possible to increase both the band-gap and quantum efficiency, resulting in UV or deep blue luminescence. Such porous SiC films exhibit a spectrally integrated photoluminescence intensity (and efficiency) which is approximately twenty (20) times higher than that which is observed from bulk SiC, thus, devices fabricated from porous SiC using existing processing techniques such as those described in U.S. Pat. Nos. 5,376,241 and 5,454,915, enable the development of semiconductor UV and blue light source and UV/blue optoelectronic devices.
The luminescence in the blue range of the spectrum (approximately 2.8 eV) can be further enhanced by passivating porous SiC with a passivation agent such as oxygen or hydrogen. Passivation enables the microcrystalline structures to satisfy the conditions for quantum confinement by preventing surface recombination at the dangling bond. Passivating agents that may be employed for this purpose include atomic hydrogen, deposited by a plasma or by an HF dip, oxygen, formed by thermal oxidation, anodically, or PECVD (plasma enhanced chemical vapor deposition) of oxygen, or any other passivating agent which will pin the dangling bond sites. The passivation exhibited by porous SiC enhanced luminescence can be utilized in the fabrication of blue semiconductor light sources such as light emitting diodes (LED's).
Initial demonstrations of room temperature visible luminescence from porous semiconductor material such as porous SiC or porous Si, created much conjecture about the mechanisms which provide visible luminescence. However, it is now generally agreed, based on considerable theoretical and experimental evidence, that at least a portion of the enhancement of the luminescence is associated with quantum structures in the porous semiconductor material. These quantum structures allow a relaxation of the momentum selection rules by confining the charge carriers spatially, thus allowing direct band-gap transitions. Additionally, it has been demonstrated in porous silicon that the quantum confinement of charge carriers increases the effective band-gap, thereby pushing it into the visible region.
It is also generally agreed that the surface chemistry in porous semiconductor materials plays an important role in luminescence. This suggests that luminescence in passivated porous semiconductor materials may have similar mechanisms as in bulk semiconductor materials like Si, which exhibit band-gap widening into the visible region when hydride species are formed on the surface. A portion of the visible luminescence of porous Si, for example, may be associated with silicon hydride (SiH). It is not positively known whether the hydrogen termination serves only to passivate the surface or whether there is a contribution to the luminescence by amorphous SiH. Nevertheless, it is very clear that silicon microcrystals having dimensions of less than 5 nm, exhibit band-gap widening and the above-described band-gap luminescence.
In terms of developing optoelectronic devices from porous semiconductor materials, some progress has been made in developing porous Si and porous Si--Ge light emitting devices. Since the oxidation rates of bulk SiC and porous SiC are much lower than that of bulk Si and porous Si, and since SiC has been identified as a material for use at high temperatures, optoelectronic devices based on porous SiC, will be much more stable over longer periods of time, and also at higher temperatures than those based on porous Si.
Accordingly, there is a need for an improved method for passivating the large internal surface area of porous semiconductors, especially porous SiC.